It is common practice in the agricultural industry to employ a tractor towing behind it an implement which receives its operating power from the tractor by means of a drive shaft connected to the power take-off (PTO) of the tractor. In order that the tractor and the towed implement can make reasonably sharp turns, it is necessary that this drive shaft be articulated, i.e., provided with U-joints or the like so that the relative angular orientations of the power take-off shaft on the tractor and the power receiving shaft on the implement can vary while the shaft continues to spin, transmitting power to the implement. It is ordinarily not possible to arrange the axes of the U-joints to coincide with the pivoting axis of the tow bar of the tractor and the draw bar of the implement so that it is necessary that the articulated shaft also telescope to permit turning.
The art has recognized for some time that it would be desirable to provide a protective outer shield for such telescoping articulated shafts because of the grave danger posed to operators and the like, who might, for example, be caught up in the machinery. However, none of the prior art approaches known fully satisfy the needs of the art which are for an inexpensive, readily manufacturable, easy to install, protective structure which would be durable in service, adaptable to a wide variety of tractors and implements, which would be attachable or removable without complex tools or difficult procedures and yet would provide total coverage of the revolving drive shaft elements.
It would clearly be desirable that such a protective guard be formed of a lightweight and inexpensive material and that of choice is clearly plastic. Metallic shield members would be unduly heavy and subject to corrosion; moreover, numerous impacts which would dent metal, but not a plastic material, can be expected to occur in the agricultural environment. The prior art shows various approaches to the use of plastics for construction of protective guards for articulated power transmission shafts, but all approaches of which the applicant is aware involve undue complexity in the manufacture of the various parts of the apparatus and substantial complexity in assembly of the device in the field. This last feature, undue complexity of assembly, is extremely important because complicated guard systems are often discarded rather than being installed and maintained properly. It must be simple enough that it can be installed by the user himself in a minimum period of time and with simple tools. Otherwise, it will not be used. In this connection it should be noted as well that typically the articulated torque transmitting shafts are permanently affixed to the implements, not the tractor. Accordingly, a farmer who hooks various implements to his tractor will want a protective guard on each of the implements. This further reinforces the requirement that the device be easy to install, as well as, of course, inexpensive to purchase.
One pertinent prior art approach to applicant's present invention is shown in U.S. Pat. No. 3,344,618 to Young. This patent shows a bell-shaped shield member extending over a yoke of a universal joint at one end of a power transmission shaft. The bell is attached to one of a pair of telescoped tubes by tabs formed in the bell. The other tube is supported by a rather complex molded plastic device which also comprises a plurality of tabs to interact with slots formed in the telescoping tubing, thus holding the assembly together. The Young tab member is annular and is slit through so that it can be sprung outwardly enough to be stretched over the shaft, thus not necessitating disassembly of the drive shaft for assembly of the device. However, it is a rather complex molding, in all dimensions, as it extends all the way around the torque transmitting shaft and has significant linear extent along it as well. This device is rather complex and expensive to manufacture. Moreover, its design requires simultaneous compression of a number of tabs before the assembly can be taken apart, which leads to undue difficulty of disassembly.
Harrington U.S. Pat. No. 2,772,550 is also relevant. This patent shows use of a pair of resilient arcuate members having an elongate portion which rides in a groove formed in a U-joint knuckle and another tab member which mates with a slot in a bell member. Telescoping tubes are stated to be similarly located, col. 2, lines 61-72, but this description is unclear and does not appear to comport with the drawings. In any event, the Harrington arrangement would require paired grooves and pairs of the arcuate members if tubes were to be similarly assembled. Moreover, the Harrington arrangement does not provide a means for attachment of the separate joint cover member directly to the shield tubing, or of a chain or the like to anchor the shield assembly against rotation, both of which possibilities are also desirable.
Other prior art expedients which are known to the applicants as being on the market involve such expedients as drilling and riveting, or adhesive connection, of relatively large diameter bell members, which guard the yokes of the universal joints, to the tubes which cover the telescoping portions of the articulated shaft. Such expedients are highly undesirable because ready disassembly of the protective guard is an important criterion of its design.
Accordingly, a need exists in the art for an improved protective guard for articulated shafting.